The invention relates to a method and apparatus for connecting two workpieces together by welding, in which the workpieces lie on supporting surfaces, are in particular butted together at their edges to form a butt joint or are disposed in an overlapping relationship, and are subjected to pressure on the opposite side to the supporting surfaces.
Various welding processes are known for joining two workpieces together, particularly for the fabrication of body components for motor vehicles. Especially worthy of note is laser welding in which the sheet metal parts are butt-welded to form compound panels. However, other welding processes too are within the scope of the invention.
A laser welding operation is usually carried out in the following manner.
The workpieces are butted together in pairs, without prepared edges, positioned without significant sideways pressure, and fixed by means of clamp elements. A first application of force to the workpieces occurs at this stage. The workpieces may then be processed, for example they may be dressed, as described in EP-A 0565846, to bring the gap within tolerance. This processing causes a second application of force to the positioning system, in which considerable lateral forces can occur. Finally, further, mainly sideways-acting force effects occur as a result of thermal stresses in the laser welding process.
It is important to find a way of blocking the lateral forces which occur. For example, if pretreatment by dressing or profiling is performed in accordance with EP-A 0565846, the average lateral force, that is to say the (horizontal) lateral force averaged over the length of the weld seam, which occurs as a result of such pretreatment can be as high as 30N/mm. Thermal stresses due to welding yield an additional average lateral force of 20N/mm which also needs to be blocked. This means that on a workpiece approximately 1600 mm long, a (vertical) clamping pressure of approximately 10 t has to be exerted. To apply such a pressure to the workpieces, heavy pressure drives and elements, and also massive structures, are required. These need costly steel constructions and larger motors if for example they are designed to be traversable. This in turn leads to structural problems in the machine itself, which reaches a size and weight that have to be taken into account when the machine is installed.
One possibility which suggests itself for making the fixing of the workpieces more secure is to provide the surfaces of the machine table and/or pressure elements with friction coatings which increase the coefficient of friction. However, only limited improvement can be gained in this way, and furthermore such friction coatings are liable to become clogged with the kind of dirt which arises in the laser welding process--greasy dust due to the subsequent cooling with oil, for example.